The measurement of substrates is used in many branches of industry. It serves for achieving a predefined accuracy in the production of substrates and for continuous quality assurance. The dimensions of substrates or of structures which are formed on substrates can be measured by coordinate measuring machines. For producing semiconductors, for example, use is made of mask blanks for producing finished masks (also referred to as reticles) used to transfer structures to silicon wafers. Both the mask blanks and the masks or the wafers are examples of substrates which must be measured highly accurately. The coordinate measuring machines used for said substrates are also referred to as registration measuring apparatuses.
For the highly accurate measurement of substrates, the latter have to be prepared.
One example of preparation is regulating the temperature of the mask. For the highly accurate measurement of substrates, the temperature within the measuring apparatus has to be constant. The measuring apparatuses are often accommodated in a climate-controlled chamber. The temperature fluctuations in said climate-controlled chamber are less than 0.5 K for example for the measurement of masks.
The substrates are stored outside the climate-controlled chamber in transport containers. The latter serve for protection against contamination and against damage. When the mask is introduced into the climate-controlled chamber, the substrate is removed from the transport container. The temperature of the substrate is then adapted to the temperature within the climate-controlled chamber by virtue of spontaneous cooling or heating of the substrate. The measurement can only be performed when the substrate has reached the temperature of the climate-controlled chamber or the temperature within the measuring apparatus. This step of regulating the temperature of the substrate is very time-consuming. A device and a method for introducing a substrate into a measuring apparatus are described in the published patent application DE19949005.
A further example of preparation for measuring a substrate is placing and aligning the substrate on a substrate holder. Measuring apparatuses are known in which substrates for measurement are placed onto a substrate holder. The position of the substrate on the substrate holder then has to be determined before the measurement of the substrate. Such methods are disclosed in the published German Patent Application DE102007033814.
Measuring apparatuses, in particular for measuring masks, are very complex and thus expensive apparatuses. A time-consuming preparation of the substrates increases the expenditure of time before the measuring apparatus is used. This gives rise to times when the measuring apparatus cannot be used since it is necessary to wait until the substrate to be measured has been prepared. Thus, very expensive measuring apparatus are not utilized optimally.